FIG. 4 shows a prior art light semiconductor device described in Applied Physics Letters Vol. 44, pp. 941.
In FIG. 4, reference numeral 1 designates an n-type substrate. Reference numerals 3, 4, and 5 respectively designate an n-type cladding layer, an active layer, and a p-type cladding layer successively disposed on the n-type substrate 1 with an intervening n.sup.+ -type buffer layer 11. Reference numeral 6 designates embedding layers disposed on the substrate 1 at both sides of the layers 3, 4, and 5. The Reference numeral 7 designates a p-type cap layer. Reference numeral 12 designates an anode electrode. Reference numerals 13, 14, and 15 respectively designate a drain, a gate, and source electrodes of a field effect transistor. Reference numeral 16 designates a photodiode.
In this prior art light-responsive semiconductor device, the light Pin incident from the outside is converted into an electric signal by photodiode 16, and is amplified by the field effect transistor comprising the source 15, the drain 13, and the gate 14. This amplified signal is transmitted to the semiconductor laser comprising the n-type cladding layer 3, the active layer 4, the p-type cladding layer 5, the embedding layers 6, the p-type cap layer 7, and the anode electrode 12. When the semiconductor laser is previously biased by an appropriate current value, a light output Pout, modulated according to the variation of the incident light Pin, is obtained.
However, in this arrangement, different components of the device, such as the semiconductor laser, the photodiode, or the field effect transistor are produced through different production processes and integrated on the same substrate. Therefore, the production is often difficult and yields are poor. Furthermore, it is often difficult to increase the area of the photodiode for receiving the incident light, thereby making it unsuitable in light information processing applications such as the processing of input information in the form of video images.
FIG. 11 shows a prior art light logic element shown in IEEE Journal of Quantum Electronics vol. QE-18, No. 9, p. 1341 to 1361, (1982) by Christoph Harder, Kam Y. Lau, and Amnon Yariv. In FIG. 11, reference numeral 16 designates an n-type GaAs substrate. Reference numeral 15 designates an n-type AlGaAs cladding layer, reference numeral 14 designates an AlGaAS active layer, and reference numeral 13 designates a p-type AlGaAs cladding layer. Reference numeral 19 designates a p-type AlGaAs current blocking layer, and reference numeral 18 designates an n-type AlGaAs current blocking layer, both provided at the sides of the active layer 14 and the cladding layers 13 and 15. Reference numeral 12 designates a Zn-diffused layer for reducing the contact resistance of the p-type electrode. Reference numeral 11 designates a SiO.sub.2 insulating film. Reference numerals 10a and 10b both designate p-side electrodes, and reference numeral 17 designates an n-side electrode.
The device is operated as follows.
At first, a slight current is made to flow through the electrode 10a in the forward direction, or a reverse bias is applied to the electrode 10a. Then, the active layer therebelow functions as a saturable absorber. That is, as shown in FIG. 12, when the light power is low, it has large absorption, and as the light power increases, the light absorption decreases.
FIG. 13 shows the relationship between the current Ib which flows through the electrode 10b and the light output L. When the current Ib is increased, the absorption at the saturable absorber is reduced gradually by the increase in the natural emission light, and when the current Ib reaches Ib.sub.1, the laser oscillation is started.
Once the laser oscillation is started, the absorption is quite small due to the strong laser light, and even when the current is reduced to some extent, the oscillation is not stopped. The oscillation only stops when the current Ib reaches Ib.sub.2. That is, when the current Ib is biased at an intermediate value between Ib.sub.1 and Ib.sub.2, this element can be used as a memory element capable of being turned on (capable of also being turned off in a case of current) by a current pulse or an incident light. This laser comprises a resonator formed by cleavage planes at the front and rear surfaces.
In this prior art light logic element, however, it is impossible to connect a number of light logic elements serially because cleavage planes are required. Furthermore, a large number of lenses and precise positioning and focusing of the respective elements are required in order to combine light logic elements.
FIG. 18 shows another prior art bistable semiconductor laser disclosed in Liu and Kamiya, Technical digest of the 10th IEEE International Semiconductor Laser Conference, Paper J-3, Kanazawa, Japan, 1986. In FIG. 18, reference numeral 16 designates an n-type InP substrate, reference numeral 15 designates an InGaAsP active layer, reference numeral 14 designates a p-type InP light confinement layer, and reference numeral 13 designates a p-type InGaAsP contact layer. Reference numeral 18 designates a p-type InP blocking layer and reference numeral 19 designates an n-type InP blocking layer. Reference numerals 12a, 12b designate p-side electrodes, and reference numeral 17 designates an n-side electrode.
The device is operated as follows.
There are provided three electrodes 12, and differentiated currents flow through the electrodes 12a at the gain region 8 and through the electrode 12b at the absorption region 9. As shown in FIG. 20, the increase in the current is accompanied by an increase in the carrier concentration. In accordance therewith the absorption coefficient is lowered and gain is quickly obtained. When a slight constant current flows through the absorption region 9 (it may be 0) and the current flowing through the gain region 8 is increased, oscillation does not occur due to the absorption at the absorption region 9 until the current becomes I.sub.1 as shown in FIG. 19. When the current exceeds I.sub.1, oscillation occurs. When oscillation starts, the absorption region 9 absorbs the laser light, and carrier density increases and the absorption coefficient is reduced as shown in FIG. 20. Accordingly, when the current is decreased gradually in this state, the oscillation stops when the current has just reached I.sub.2. In this prior art device, a bistable property is obtained.
Herein, the rise and decay times for rising up and falling down of such a laser is about 200 pico seconds at minimum. But this is not fast enough.
A quantum well structure is usually used for the active layer in order to enhance the operation speed in a semiconductor laser. In such a case, however, the absorption coefficient of the quantum well structure in the current non-injecting state is low at the wavelength at which the gain coefficient in the current injected state becomes the highest as shown in FIG. 2, and it is not large enough to exhibit bistable operation.
The prior art bistable semiconductor laser has drawbacks in that operation speed is limited and the oscillation wavelength is unstable.